1. Field of the Invention
The present invention relates to a fuel injection control system for an internal combustion engine having an intelligent timer.
2. Description of the Related Art
Control systems using a microcomputer for controlling an internal combustion engine are known. In order to reduce the computational load to or number of processing steps of performed by a CPU (central processing unit) in a microcomputer, some recent control systems are equipped with an intelligent timer (hereinafter referred to as TPU) in addition to the CPU to control the engine. The TPU is a timer having control functions, such as outputting a desired signal when a predetermined point in time is reached. Specifically, with only the timing for starting a fuel injection, a fuel injection time, and the like obtained by a calculation performed by the CPU set in a parameter RAM in the TPU, the TPU independently performs subsequent injection executing processes, such as driving injectors for injecting fuel for a predetermined time at predetermined angular displacement or position without requiring processing by the CPU.
In such conventional systems, however, the TPU can perform only processes which are in synchronism with rotational angle signals such as a process of starting injections based upon rotational angle signals at predetermined intervals. Therefore, the TPU has not been practical because it has been unable to perform asynchronous injections, wherein fuel is injected at times which are not in synchronism with rotational angle signals.
Although asynchronous injections can be performed by way of control of the output of the TPU performed by the CPU, asynchronous injection times must be controlled by the CPU in such a case. As a result, additional calculation processes are required for the CPU to control asynchronous injections, notwithstanding the fact that the TPU is provided to reduce the calculation processes of the CPU. Further, a timer for asynchronous injections must be separately prepared, which complicates the system configuration.